Sorry to say brother, but insulting and slandering is from the teachings of Shiism not Islam. Just check out this shia hadeeth: Imam Al-sajjad (as) said: If you see people of suspicion and innovation – other than shias or new shia – then show disownment from them and abuse them much, backbit them, make false accusations on them – that is, backbite them by attributing lies on them and make false accusations on them (‘Buhtaan’) …(tanbiah al-khawatir v.2 p.162 – wasael al-shia v.11 p. 508 – Nahj al-intisaar p.152)

Imam Al sajjad(as)’ll never give such instruction,its not befitting of an infallable Imam to instruct his shia to act contrary to the teaching of Islam.2ndly,this is just a Hadith to which is subject to scrutiny/probe.The way the sunni especially,wahabi/salafi view Hadith differs greatly from shia.